Assuming you have RVM installed correctly:
1) Open Terminal.
2) Type rvm list
to see which versions of Ruby you have installed.
3) Type rvm use ruby-2.3.1
if you want to use Ruby 2.3.1.
4) Type rvm gemset create Rails500
to create a gemset for Rails 5.0.0
5) Type rvm gemset use Rails500
to make Rails 5.0 your current gemset
6) Type gem install rails -v 5.0.0
to install Rails 5 to your Rails500 gemset
7) Type bundle install
bring everything up to date.
Now you have Rails 5.0 running under a Rails500 gemset against Ruby 2.3.1. Remember to use ruby-2.3.1
and use gemset Rails500
whenever you start a new session in Terminal if you want to maintain multiple versions of Ruby and multiple gemsets.
To make a version of Ruby default, type the following into Terminal:
rvm --default use 2.1.1
To keep a default gemset, just rvm gemset use default
and make sure you install your gems into that default
gemset by use
-ing it before installation.
This covers 80% of everything you'll need to know about RVM.
Also, remember to never install rbenv, because it's not compatible with RVM -- stick with one or the other.
EDIT:
It seems you're having problems with your Terminal settings as well.
1) Open Terminal
2) From the Terminal menu, select Preferences
3) Select the Command (complete path) radio button
4) Make sure the text field beneath the radio button reads /bin/bash
5) Close the Preferences dialog and restart Terminal
6) Try using Ruby 2.3.1 via RVM